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People with allergies know that their sniffling and sneezing is allergen-induced, but allergies don't always present themselves with such typical symptoms. Sometimes allergic reactions can cause fatigue, headaches — or even depression. While not everyone agrees there is an allergy link to these conditions, there is evidence that strongly suggests it. Here's what's been found, and how to control your allergies to alleviate each problem.

Chronic fatigue syndrome
If you've been experiencing extreme exhaustion for 6 months or more and your doctor hasn't been able to identify the cause, you might have chronic fatigue syndrome (CFS). And if you do have CFS, allergies could be playing an important role, says Leo Galland, MD, director of the Foundation for Integrated Medicine in New York City and author of "Power Healing."

Though no one knows exactly what causes CFS, researchers have found that more than half of the people with CFS they've studied also have allergies. "I believe that being an allergic individual predisposes you to chronic fatigue syndrome," says Galland. "Chronic fatigue syndrome seems to be associated with an over-reactivity of certain parts of the immune system, which is similar to what we see in people with allergies."

And when allergies are part of the cause, treating the allergies can be a part of the cure. "I've found that close to three-fourths of my patients will find their fatigue improves when their allergies improve," Galland notes. This improvement varies widely, but sometimes it can be dramatic. "There have been some patients in whom disabling chronic fatigue totally goes away when their food allergies were treated," he reports.

Environmental allergens and irritants can also compound your fatigue. "Mold allergy is an important cause of fatigue and muscle aches. A significant proportion of people with chronic fatigue syndrome and fibromyalgia have mold sensitivity," says Galland. If you have been diagnosed with CFS, you should see an allergist to get a thorough evaluation for allergies, he advises.

Depression
Scientists acknowledge that allergens can contribute to mood alterations. In a 3-year study of 36 people with allergies, Paul S. Marshall, PhD, a psychologist at Hennepin County Medical Center in Minneapolis, found that 69 percent reported feeling more irritable when their allergies flared up; 63 percent reported more fatigue; 41 percent said that they had difficulty staying awake; and 31 percent reported feeling "sad." So the idea that allergies might exacerbate mild depression in a few people who have other allergic symptoms isn't that far-fetched to some researchers.

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"My guess is if there is a connection, it is not true for all people with allergies or all people with depression," says Marianne Wamboldt, associate professor of psychiatry at the University of Colorado Health Sciences Center, both in Denver. "But for a small subset of individuals, it does appear that these conditions do seem to exacerbate each other."

In her analysis of the incidence of allergies and depression among more than 7,000 Finnish twins, Wamboldt found that genetics may explain 10 percent of the connection between allergic disorders and depression. Other studies have suggested that people who have undergone allergy testing or received allergy shots are two to three times more likely to be diagnosed with major depression at some point in their lives, Marshall reports.

"Traditionally, allergists have said that depression is the result of the allergic symptoms — you're not sleeping well or not breathing well because your nose is stuffed up. Now we have a good deal of evidence that suggests there is a direct biochemical process going on in at least a few people," Marshall says.

"Some researchers are beginning to suspect that some types of depression may be triggered by inflammatory reactions in the body," Wamboldt says. But this theory is far from proven, and for now, there is no single treatment that will relieve both allergies and depression, Marshall notes.

"All we can really do is treat the depression with therapy and/or antidepressant medications and treat the allergies with shots, antihistamines, and avoidance of the allergens," Marshall says.

Rates of women who are opting for preventive mastectomies, such as Angeline Jolie, have increased by an estimated 50 percent in recent years, experts say. But many doctors are puzzled because the operation doesn't carry a 100 percent guarantee, it's major surgery -- and women have other options, from a once-a-day pill to careful monitoring.

Sinus and migraine headaches
Some doctors believe that headaches are a common symptom of allergies. In fact, up to 15 percent of Americans who have allergies may endure headaches triggered by pollen, food, and other allergens, according to Dennis Gersten, MD, who practices psychiatry and nutritional medicine in Solana Beach, CA, and is the author of "Are You Getting Enlightened or Losing Your Mind?"

"Most headaches are related to tension, hormonal fluctuations, skipped meals, drops in blood sugar, vision difficulties, and nutritional deficiencies. But if all of those things have been ruled out, then I think allergies are a definite possibility," Gersten says.

Not all doctors agree about the connection between headaches — especially migraines — and allergies. Russell Roby, MD, founder of the Online Allergy Center, acknowledges allergies are rarely suspects in tension headaches. These are almost invariably caused by stress or fatigue and often feel as if a tight band is wrapped around your head. But sinus and migraine headaches are a different story.

When you breathe in an allergen, such as ragweed, it can trigger swelling and obstruction of the nasal passages. As a result of this reaction, the sinuses are unable to drain, and this increases pressure throughout the skull, which triggers a headache, says Harold Nelson, MD, senior staff physician at the National Jewish Medical and Research Center in Denver. In fact, this condition, known as sinusitis, may be one of the most frequent causes of headaches in people with allergies. Symptoms include pressure in the forehead, cheeks, and behind the eyes, along with tooth pain and yellow or green nasal discharge.

The relationship between allergies and migraine is a bit more complex, notes Nelson. But basically, researchers suspect that cells in the immune system, which are sensitive to particular allergens, release chemicals that encourage blood vessels in the head to swell, inciting a migraine. Certain foods, such as chocolate and red wine, are notorious for triggering migraines in some people.

To muffle allergy-related headaches, you'll need to outwit your immune system's natural defenses. "I find that anytime my patients are having allergy problems, food is often part of the problem. It is unusual for food to be the only cause of a problem like headaches, but it does happen," Roby says. He recommends a 5-day food elimination diet to find the culprits. "This 5-day plan does not require medical supervision in a healthy adult. If one has health problems, advice and consent from your physician are advised before trying these suggestions."

For at least 5 days, eliminate the following common headache triggers:

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After 5 days, resume eating these foods one at a time and note your body's reaction to them. In particular, keep track of your weight. "When patients eat something they are allergic to, they swell. This swelling is entirely due to water retention," Roby explains.

Paradoxically, the foods we are most emotionally attached to often are the ones causing the allergy. After a round on the elimination diet, Roby advises his patients to reintroduce whatever they miss the most. "I do this so that the patient gets a clear signal that the headaches are indeed related to foods."

Yeast infections
"Just like people have hay fever [allergic rhinitis], you can become allergic to particular molds and yeasts," says Elson Haas, MD, medical director and founder of the Preventive Medical Center of Marin in San Rafael, CA.

Believers in the candida-allergy connection theorize that once Candida albicans starts multiplying, the yeast changes shape into its mycelial fungal form. In this form, candida grows long, rootlike structures that dig into the intestinal walls and act as pipelines for candida, pumping toxins and allergens generated by the fungus into the bloodstream.

Probably the most common form of candidiasis — and the one that all doctors agree is legitimate — is vaginitis, more commonly known as a yeast infection. Seventy-five percent of women will experience at least one during their lifetime.

Candidiasis hypersensitivity usually can be treated without antibiotics. In fact, the first thing that Richard Layton, MD, a physician in specialized pediatrics and allergy and preventative medicine in Towson, MD, does when he sees a potential case of candidiasis isn't fill out prescriptions; instead, he gets that person back on a healthy diet. Here are the three main steps to take:

Stop eating sweets. The candida yeast thrives on sugar, so the best way to kill it off is to avoid sweets.

Avoid yeasty foods (bread, cheese, and mushrooms). If you're sensitive to the candida yeast, it's likely that you will be sensitive to other molds as well.

Eat more yogurt. Live-culture yogurt contains Lactobacillus acidophilus, a type of "good" bacteria in the body. Since an adequate amount of this bacteria maintains yeast at a normal level, adding a little more plain, unsweetened yogurt to your diet certainly will not hurt.